Since plastic lenses are lighter, less fragile, and tintable, as compared with inorganic lenses, therefore, in recent years, the application of plastic lenses to optical elements such as spectacles lenses, camera lenses and the like has increased rapidly.
In particular, spectacles lenses have been required to have various features including optical properties such as (a) high refractive index and (b) low dispersion (high Abbe's number), and being (c) excellent in heat resistance, (d) excellent in impact resistance, (e) easily tintable, (f) excellent in working properties such as cutting properties, or the like. Accordingly, various resin materials for lenses have been developed and used until now.
Representative examples of those can be mentioned by polythiourethane-based resins (Patent Documents 1 and 2). Among the polythiourethane-based resins, a polythiourethane-based resin obtained by using a polyisocyanate compound represented by Formula (1) (Patent Document 3) is used to provide a plastic lens having a high refractive index and low dispersion property, and exhibiting excellent heat resistance and impact resistance, thus it has been widely used for spectacles lenses.

In Patent Document 4, there disclosed a plastic lens prepared by using a polyurethane/thiourethane-based resin, where the lens has a refractive index of 1.545 to 1.556 and excellent impact resistance, and is obtained by the reaction between diisocyanate oligomer obtained by reacting specific amounts of isophorone diisocyanate, hexamethylene diisocyanate, and a divalent alcohol at a temperature between 40° C. and 70° C., and specific polythiol.
In addition, in the case of industrially preparing lenses, generally a large quantity of monomeric mixture is prepared at once, filtered thorough a fine filter of about 1 to 3 micron over number of hours to remove fine insoluble fractions and dust, and then the resultant is injected into a mold to be cured.
Therefore, there has been demanded a simple preparation of monomeric mixture, little heat generation upon blending, low viscosity of the mixture, low rate in viscosity increase, excellent filtration property and the like, from the viewpoints of handleability and productivity.
However, when carrying out the reaction between a polyisocyanate compound, a polythiol compound, and a polyol compound, during the preparation of the lens prepared by using a polyurethane/thiourethane-based resin, there has been a case where the reaction between an isocyanate group and a thiol group is accelerated by the heat caused from the reaction between a hydroxy group and an isocyanate group because the reaction of an isocyanate group with a hydroxy group proceeds faster than the reaction of an isocyanate group with a thiol group, thus may result in a runaway reaction. In consequent, depending on the conditions, abrupt heat generation by monomer upon blending, high viscosity, thereby being difficult to carry out injection into a mold, have come up as the concerns. In addition, there is room for improvement in blending large amount of monomer. Accordingly, there has been demanded a blending method by which a large heat generation is not occurred even if a polyisocyanate compound, a polythiol compound, and a polyol compound are blended in large quantity, and the temperature is sufficiently controlled.
In the aforementioned Patent Document 4, in order to avoid such runaway reaction, there disclosed a method which includes reacting a mixture of isophorone diisocyanate and hexamethylene diisocyanate with ethylene glycol or propylene glycol as s particularly preferable divalent alcohol compound at a temperature between 40° C. and 70° C. for 19 hours, so as to first obtain diisocyanate oligomer, and then carrying out a polymerization with a thiol compound.
In Patent Document 5, there disclosed that a molded product having high impact strength can be obtained by polymerizing an isocyanate compound with a specific structure, an aliphatic diol compound, and a polythiol compound with a specific structure.
However, in the case of employing tris(6-isocyanatohexyl)isocyanulate which is one of the isocyanate compounds with a specific structure, since it is a trifunctional compound, initial viscosity thereof is high and this viscosity increases further while proceeding the polymerization, thereby leading to difficulty in blending monomer in large quantity.    [Patent Document 1] Japanese Patent Laid-open No. 2-270859    [Patent Document 2] Japanese Patent Laid-open No. 252207    [Patent Document 3] Japanese Patent Laid-open No. 3-124722    [Patent Document 4] Korean Patent Laid-open No. 2003-078494    [Patent Document 5] Pamphlet of PCT Publication No. 2004/108787